White light-emitting diodes (LEDs) exhibit high efficiency, long lifetimes, less environmental impact, absence of mercury, short response times, applicability in final products of various sizes, and many more favorable properties. They are gaining attention as backlight sources for liquid crystal displays, computer notebook monitors, cell phone screens, and in general lighting.
By combining red, green, and blue emitting phosphors with an near UV LED, which typically emits light at a wavelength ranging from 280 to 400 nm, as a primary light source, it is possible to obtain a tri-color white LED with better luminescence strength and superior white color.
Typically, a red, a green, and a blue emitting phosphor are first mixed in a suitable resin. The resultant gel is then provided on a UV-LED chip or a near UV-LED chip, and hardened by UV irradiation, annealing, or similar processes. The phosphor mixture in the resin should be as homogeneously dispersed as possible in order to observe an even, white color, while looking at the chip from all angles. However, it is still difficult to obtain a uniform distribution of the different phosphors in the resin because of their different particle sizes, shapes and/or their density in the resin. Hence, it is advantageous to use less than three phosphors.
However, even by using a mixture of two phosphors, in order to produce white LEDs using UV or near UV-LEDs, it is still difficult to uniformly mix phosphors having different sizes, particle shapes, and densities in the resin. Moreover, the phosphors should not be excited by a wavelength located in the visible range. For instance, if the emission spectrum of the green phosphor overlaps with the excitation spectrum of the red phosphor, then color tuning becomes difficult. Additionally, if a mixture of two or more phosphors is used to produce white LEDs using a blue emitting LED as the primary light source, the excitation wavelength of each phosphor should efficiently overlap with the blue emission wavelength of the LED.
As known to the expert, white LEDs can be also obtained by adding a yellow emitting phosphor to a blue light emitting LED.
Typically, the yellow phosphor used in such applications is yttrium aluminum garnet activated by Ce3+, Y3Al5O12:Ce3+, (YAG:Ce) as described for example in S. Nakamura, G. Fasol, “The Blue Laser Diode”, (1997) p. 343. Also some ortho-silicates M2SiO4:Eu2+(M=Ca, Sr, Ba) are suggested, to be used as yellow-orange emitters, as disclosed for example in G. Blasse, et al., Philips Res. Rep., 23 (1968) 189. Moreover various nitrides and oxy-nitrides, doped with divalent europium or trivalent cerium ions, such as M2Si5N8:Eu2+(M=Sr, Ba), may be utilized as described in H. A. Höppe, H. Lutz, P. Morys, W. Schnick, A. Seilmeier, J. Phys. Chem. Solids 61 (2000) 2001.
However, the aforementioned materials suffer from the fact that the spectral region covered, is not sufficient to produce warm white light. Moreover, the nitrides and oxy-nitrides are expensive host lattices, due to the physical properties of the starting nitride materials, namely hygroscopicity and sensitivity to ambient atmosphere.
Accordingly, there is still room for improvements and modern luminescent materials should, preferably exhibit one or more of the following properties:                exhibit high colour rendering indices (CRI),        exhibit a broad emission band in the range of the VIS-light, especially in the red range of the spectra,        are effectively excitable by an blue light or near UV emitting primary light source,        exhibit broad excitation bands,        exhibit high quantum yields,        exhibit high phase purities,        exhibit high efficiency over a prolonged period of use,        have high chemical stability, preferably against humidity or moisture,        exhibit higher thermal quenching resistivity, and        are obtainable by method of production, which should be cost efficient and especially suitable for a mass production process.        
In view of the cited prior art and the above-mentioned requirements on modern luminescent materials, there is still a considerable demand for alternative materials, which preferably do not exhibit the drawbacks of available phosphors of prior art or even if do so, to a less extent.